1. Field of Invention
This invention relates to the field of firearms, specifically accessories and mounting devices for firearms.
2. Prior Art
Several years ago the military started the SCAR program to select a weapon which would replace all rifles currently utilizing a direct impingement operating system in use by the military. In short, a replacement of the M16/M4 rifle was being sought.
To explain the difference in the two systems, both direct gas impingement rifles and piston driven rifles have a hole toward the front end of the barrel. As the cartridge is fired, burning propellant causes the bullet to move down the bore of the rifle. The propellant burns, generating expanding gases thereby increasing the pressure behind the bullet. As the bullet passes the hole in the barrel, some of the gas is tapped off. This is where the direct gas impingement and piston driven rifles become different.
In a direct impingement rifle the gas is directed by means of a long tube back into the receiver of the rifle. There is a cupped bolt carrier key which the tube fits into. The force of the gas pushes back the bolt carrier and reciprocates the rifle's self loading action. The disadvantage of this is dirty gases and carbon residues are deposited into the rifle's action and with sustained fire can lead to eventual failure as the close tolerance parts are caked with carbon. The long trip the gas must make before imparting its force also allows much of the gas to leak out of the system if parts are not mated exactly.
Rifles and carbines utilizing a gas piston system offer the user a more reliable, robust, controllable, cleaner operating system. As the gas is tapped off the barrel, it is immediately met by a gas piston. The gas drives the piston back which pushes a rod that actuates the action of the rifle. The gas piston uses all of the tapped gas to generate the rear momentum of the piston. Once it has moved back far enough with enough momentum to actuate the mechanism of the rifle, all excess gases are rapidly vented off. The rapid venting of all excess gases makes for an efficient and clean, self regulating system since about 5% of the volume of gas is actually used to operate the firearm. None of the hot, dirty gas or carbon enters the moving parts located in the receiver of the host firearm.
In recent years there has been industry-wide interest to incorporate a gas piston system into the M16/M4 type rifle and its variants. The push for this substantial change is due to the shortcomings which the direct gas impingement M16/M4 type rifle has shown in sandy environments and after prolonged use.
Traditionally the M16/M4 type rifle utilized a gas tube, well known in the prior art, which has a curve along its length thus allowing for the current generation of rail adaptor systems (RAS's) system and hand guards to function. Gas piston systems have to be relatively straight along the entire length of the piston utilized. This straight piston and related components necessitated the development of new methods for the attachment of a RAS.
The advantages of a gas piston system are readily apparent to those familiar with the prior art. Unfortunately the new system left the traditional M16/M4 rifle equipped with a gas piston system deficient in several areas in which it previously was proficient. Examples:                1. Lack of an ability to utilize existing free float rails, and tubes. If a traditional free float tube or rail were utilized it would prevent the users from accessing the gas operating system above the barrel for cleaning purposes. Free float tubes and rails are necessary to prevent pressure from bi-pods and other accessories from pushing the hand guard, tube or rail into contact with the barrel thus compromising the weapon's accuracy potential.        2. Also recognized is the inability of the user to mount accessories which would traditionally be attached to rail interface systems. A rail interface system traditionally consists of a closed tube secured at or near the receiver of the firearm with a variety of accessory mounting rails located about the periphery of the tube. Currently available rails do not interface with a gas system and/or do not allow for access to the operating system for maintenance.        3. Other rails currently available do not facilitate the user's access to the gas piston operating system for cleaning without the total removal of the free float device. Removal of the free floating rail in its entirety to clean the operating system of the weapon will result in a loss of zero, or the ability of the weapon to direct a discharged bullet to a desired location would be compromised. In effect a military user would be prohibited from cleaning his/her weapon while in a combat environment.        4. Many currently available rail adaptor systems are difficult to install requiring an armorer or gunsmith with special tools to do the work.        
Previous rail adaptor systems (RAS's) such as shown in U.S. Pat. No. 5,826,363 (Douglas Olson) have consisted of an aluminum tube which replaces the hand guards. The tube has a series of standard 1913 Picatinny slotted rails at the 3, 6, 9, and 12 o'clock positions along the longitudinal axis of the bore. These Picatinny slotted rails allow for the use and alignment of various accessories with the axis of the bore, an example being sighting equipment to increase the functional ability of the host firearm. The RAS allows the end user to customize the firearm for a mission or make employment of the rifle more ergonomic, effective and/or comfortable. A variety of RAS systems have been developed, predominantly for M16/M4 rifles and clones thereof. While not the first, U.S. Pat. No. 5,826,363 (Douglas Olson) is the first truly useful RAS system in that it is capable of mounting and carrying the modern load of combat accessories.
Examination of the prior art puts these systems into different categories. There are “conventional” designs that work with the standard hand guard retaining ring on the rifle (delta ring) and simply replace the hand guards. These systems are usually found to be two-piece designs, two semi-circular halves making up a tube. These designs transmit the weight of the accessories to the barrel of the rifle, changing the point of impact of the bullet. They also transmit external forces such as pressure from the user's hand, or force exerted by the weight of the rifle resting on a fixed support like a sandbag or bi-pod. Since these designs are mounted directly to the barrel and gas block, they transmit the heat from the barrel through the highly conductive aluminum alloy to the hand of the users.
The other general category of RAS's are one or two piece designs that use a proprietary system to clamp onto the rifle without contact with the barrel forward of the barrel nut allowing the barrel to “free float” and thus not affecting the point of impact by outside forces or the weight of the accessories mounted on it.
Once mounted, neither type of RAS is readily removable without some type of mechanical fixture and tool. None have provisions to allow easy access to parts underneath the top rail of the hand guard, leaving the user to abandon maintenance of the parts once the RAS is installed.
Mil. Std. 1913 covers specifications of the longitudinal rails affixed, molded, cast, extruded onto, or as part of the Rail Accessory System. The standard exists so that a host of manufacturers can design and manufacture accessories to easily affix to the rail and thus the rifle.
No prior art documents describe or illustrate a RAS which is readily adaptable to either a gas piston or direct gas impingement rifle design. None have made provision for a removable top rail to access the gas system for maintenance while free floating the barrel and providing clearance and ventilation for the gas system parts. There is no evidence of an RAS being conceived that could switch between rifles of one family to another with the use of simple conversion parts. No currently available RAS may be installed without a series of specialized tools.